The present invention concerns a method of obtaining calorimetric values from a printed image produced by a printing press for use in regulating coloration by a printing press.
Coloration is regulated to advantage on modern printing presses, in offset printing in particular, by color difference control. A typical color difference control method is described in European Patent No. 228,347 B2 and German Patent No. 195 15 499 C2, for example. In this method, a printed sheet printed with the printing press is measured calorimetrically in numerous test areas with regard to a selected color coordinate system. Using the color coordinates thus obtained, color difference vectors with respect to theoretical color coordinates based on the same color coordinate system are calculated. These color difference vectors are converted to change in layer thickness vectors with the help of calorimetric value gradients, and the ink management of the printing press is regulated on the basis of the change in layer thickness vectors converted from the color difference vectors. Fields of color control strips printed along with the actual printed image are used as test areas.
Meanwhile, scanning devices known as scanners have become known in general, making it possible to perform calorimetric or spectrophotometric measurements of the total image content of a printed sheet in a large number of relatively small pixels at a reasonable cost within a very short period of time. These scanners offer the essential prerequisites in terms of measurement technology for using not only concurrently printed test strips for ink management control on a printing press, but also the color information from all pixels of the entire actual printed image for this purpose.
For calculating the input quantities for the control device of the inking devices of the printing press, the color difference vectors of the scanned pixels of the printed sheet are analyzed in comparison with the corresponding pixels of an OK sheet or another reference, either directly or by the methods described in European Patent No. 228 347 B2. This direct analysis of color difference vectors as such is admissible if only the fields of color control strips printed together with the actual printed image or only a few specially selected test areas from the actual printed image are used as test areas. However, if the total image content, i.e., all or most of the pixels of the printed image, is to be used for analysis, this procedure fails entirely because the print conditions in the individual pixels normally differ greatly, or at least it leads to faulty control. The color difference vectors of the individual pixels thus cannot generally be used as such as a basis for further processing with in-image measurement.
Against the background of this related art, an object of the present invention is to improve upon a method of the generic type to the extent that it leads to colorimetric values suitable for inking control on a printing press even with in-image measurement.
The present invention provides a method of obtaining calorimetric values intended for regulating inking on a printing press from a printed image produced by the printing press, the printed image (3) being measured calorimetrically in a plurality of pixels (4) with regard to a selected color coordinate system; color difference vectors (xcex94F) with respect to predetermined reference color vectors based on the same color coordinate system, predefined or determined from a reference printed sheet (3) being calculated from the color vectors (F) thus obtained for each pixel (4), and these color difference vectors (xcex94F) being used as the colorimetric values (FMW) intended for regulating inking by the printing press (1). The present invention is characterized in that the color difference vectors (xcex94F) of the pixels (4) are multiplied by a weighting factor (g1; g2) which is determined from the ink coverage of the pixels (4) and/or the color differences between the pixels (4) and their neighboring pixels (4).
Especially advantageous embodiments and refinements include: (a) that the ink coverages of each pixel (4) with respect to the printing inks involved are determined; the weighting factor (g1) of a pixel (4) has a maximum value, in particular a value of 1, when the average or one of the ink coverages of the pixel (4) exceeds a predetermined first threshold value, in particular a value of 10%, and otherwise the weighting factor (g1) has a smaller value, in particular a value of 0; (b) that the ink coverages of each pixel (4) are determined with respect to the printing inks involved; the weighting factor (g1) of a pixel (4) has a maximum value, in particular a value of 1, when the sum of the ink coverages of the respective pixel (4) falls below a predetermined threshold value, in particular a value of 250, and otherwise the weighting factor (g1) has a smaller value, in particular a value of 0; (c) that the color difference with respect to an unprinted location on the printed sheet (3) is determined for each pixel (4); the weighting factor (g1) of a pixel (4) has a value of 1 when the color difference between the pixel (4) and the unprinted location exceeds a predetermined second threshold value, in particular a value of 5, and the weighting factor (g1) otherwise has a smaller value, in particular a value of 0; (d) that for each pixel (4) the color differences in comparison with its immediately adjacent pixels (4) are determined; the weighting factor (g2) of a pixel (4) has a maximum value, in particular a value of 1, when the sum of the color differences falls below a predetermined third threshold value, in particular a value of 8, and the weighting factor (g2) otherwise has a smaller value approaching 0 with an increasing sum of the color differences; (e) that for each pixel (4) a weighting factor (g) is determined, obtained by multiplication of the weighting factor (g2) calculated on the basis of the color differences between the pixel (4) and its neighboring pixels (4) by the weighting factor (g1) calculated on the basis of the ink coverages of or the color difference between the pixel (4) and an unprinted location on the printed sheet (3); and (f) that the ink coverage difference between a pixel (4) and its neighboring pixels (4) is used to determine the weighting factor (g2).